Factual Information 2015:1.9 Communications/1.9.4 ACARS
This page contains an extract from MH370/01/15 Factual Information which accompanied the (first) Interim Statement released by The Malaysian ICAO Annex 13 Safety Investigation Team for MH370 on 8th March 2015.
1.9.4 Aircraft Communications Addressing and Reporting System (ACARS)
ACARS is a digital data-link system that manages flight plan and maintenance data between the aircraft and the Ground Service Provider (GSP) by using radio i.e. VHF or satellite communications (SATCOM) as shown in Figure 1.9C below. ACARS provides message communication between aircraft and its base (ground). The following messages are transmitted:
a) Out of the gate, Off the ground, On the ground, and Into the gate (OOOI) events
i. Out of the gate event: Departure from the gate with all doors closed and parking brake released
ii. Off the ground event: Take-off with the nose gear squat switch extended.
iii. On the ground event: Touch down with the nose gear squat switch compressed.
iv. Into the gate event: Parked at the gate with the parking brake set and the door open.
b) Flight plans: ACARS interfaces with Flight Management Systems (FMS) acting as the communication system for flight plans to be sent from the ground to the FMS. This enables the aircraft to update the FMS while in flight, and allows the flight crew to evaluate the alternative flight plans including the status of connecting flights.
c) Weather information: ACARS interfaces with FMS, acting as the communication system for weather information to be sent from the ground to the FMS. This enables the aircraft to update the FMS while in flight, and allows the flight crew to evaluate new weather conditions.
d) Equipment health: ACARS is used to send information from the aircraft to ground stations about the conditions of various aircraft systems and sensors in real-time. Maintenance faults and abnormal events are also transmitted to ground stations along with detailed messages, which are used by MAS for monitoring equipment health, and to better plan the repair and maintenance activities.
e) Aircraft positions which provide latitude and longitude, altitude, speed, total air temperature total remaining fuel, wind direction and speed and heading.
f) Engine performance data which provide engine data during take-off, climb, cruise and approach.
ACARS interfaces with interactive display units in the cockpit, which flight crews can use to send and receive technical messages and reports to or from ground stations, such as a request for weather information or clearances or the status of connecting flights. The response from the ground station is received on the aircraft via ACARS as well.
The ACARS communicates through either SATCOM or VHF. The VHF transceiver can also be used for voice transmission when ACARS is switched from the data mode to the voice mode.
In the event that the aircraft ACARS unit has been silent for longer than a pre-set time interval, the ground station can ping the aircraft (directly or via satellite). A ping response indicates a healthy ACARS communication.
Pre-set time interval for MAS B777 is 30 minutes. When the aircraft ACARS is silent for more than 30 minutes, MAS Operation Control Centre (OCC) will send a text message via ACARS to the cockpit or will call the cockpit via SATCOM.
Source: Malaysian ICAO Annex 13 Safety Investigation Team for MH370, 8 March 2015, Factual Information MH370/01/15
The Factual Information was updated in 2018 by the Safety Investigation Report MH370/01/2018 which added new content but did not include all of the previous data.